The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics1[CC-BY]
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Topical Review The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics1[CC-BY] Mareike Jezek and Michael R. Blatt* Laboratory of Plant Physiology and Biophysics, University of Glasgow, Glasgow G12 8QQ, United Kingdom ORCID IDs: 0000-0002-7460-0792 (M.J.); 0000-0003-1361-4645 (M.R.B.). Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO2 entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields. Stomata are pores that form across the epidermal cell layer of plant leaves and stems. They connect the inner air space of these organs with the atmosphere, thereby serving as the major route for gaseous exchange, bypassing the otherwise impermeable cuticle that forms on the outer epidermal surface. Stomata respond to environmental and endogenous (chemical and hy- draulic) signals, opening and closing the pore in order to satisfy the needs of the mesophyll cells for CO2 in photosynthesis while limiting water loss via transpira- tion to the atmosphere. In the light, stomata may reduce photosynthetic rates by 50% and more when water supply is limiting (Lawson and Blatt, 2014; Vialet- Chabrand et al., 2017). They have a major impact on global water and carbon cycles. Transpiration by crops has been a key factor in global atmospheric modeling and weather prediction for over a quarter of a century (Beljaars et al., 1996; Berry et al., 2010). Today, stomatal transpiration is widely recognized to lie at the center of the crisis in water availability and crop production now expected over the next 20 to 30 years. Water use around the world has increased 6-fold in the past 100 years, 1 This work was supported by the Biotechnology and Biological Sciences Research Council (grant nos. BB/N01832X/1, BB/L001276/1, BB/M01133X/1, BB/M001601/1, and BB/L019205/1), the Euro- pean Union (grant no. NEURICE 678168), and the Leverhulme Trust, the Royal Society, and the John Simon Guggenheim Memorial Foun- dation to M.R.B. * Address correspondence to [email protected]. M.J. assembled the tables; M.J. and M.R.B. prepared the figures and wrote the article. [CC-BY]Article free via Creative Commons CC-BY 4.0 license. www.plantphysiol.org/cgi/doi/10.1104/pp.16.01949 Ò Plant Physiology , June 2017, Vol. 174, pp. 487–519, www.plantphysiol.org Ó 2017 The author(s). All Rights Reserved. 487 Downloaded from on July 10, 2017 - Published by www.plantphysiol.org Copyright © 2017 American Society of Plant Biologists. All rights reserved. Jezek and Blatt twice as fast as the human population, and is expected transporters, it has been possible to connect gene to to double again before 2030, driven mainly by agricul- function through heterologous expression and analysis ture and irrigation (UNESCO, 2015). Indeed, there are in isolation. This same strategy has been used to dissect some very basic reasons for pursuing an understanding macromolecular protein complexes regulating several of how stomata work. K+ channels (Honsbein et al., 2009; Grefen et al., 2015) Stomata attracted the attention of early microsco- and to reconstruct speculative phosphorylation cas- pists, including Grew (1682), who described stomata as cades (Geiger et al., 2009, 2011). Imaging techniques breathing holes on the surface of plant leaves. de combined with voltage clamp studies have shown how Candolle (1827) first confirmed that stomatal apertures are individual transporters are regulated in vivo by cyto- 2+ 2+ variable, but it was only later that von Mohl (1856) solic free [Ca ] ([Ca ]i) and pH (pHi; Thiel et al., 1993; would appreciate the importance of turgor in driving Grabov and Blatt, 1998; Hamilton et al., 2000; Loro these changes. A number of observations recognizable et al., 2012). Site-directed mutation, complementation today followed the advent of the diffusion porometer studies, and structural analysis have uncovered the that enabled measurement of the resistance of the leaf to molecular mechanics of channel gating (Riedelsberger gaseous flow (Darwin and Pertz, 1911). These included et al., 2010; Lefoulon et al., 2014) and early events of transient movements (Darwin, 1916; Knight, 1916), ABA perception and signaling (Garcia-Mata et al., 2003; midday closure (Loftfield, 1921), and the effects of Melcher et al., 2009; Cutler et al., 2010; Wang et al., 2013; drought (Laidlaw and Knight, 1916). Freudenberger Eisenach and Di Angeli, 2017; Inoue and Kinoshita, (1940) and Heath (1948) showed that CO2 within the 2017). leaf air space was important in regulating aperture, and The actions of other hormones such as auxin (Blatt Wilson (1948) established the importance for stomatal and Thiel, 1994; Lohse and Hedrich, 1995), of light and movements of the vapor pressure difference between CO2 (Negi et al., 2008; Kim et al., 2010; Xue et al., 2011; inside and outside the leaf. Kinoshita, 2017), and of plant pathogens (Melotto et al., Stomata were inextricably bound up with the plant 2008) have not been neglected (Melotto et al., 2017). hormone abscisic acid (ABA) when Wright and Hiron Resolving the interface between transport and carbo- (1969) at Wye College in the United Kingdom and hydrate metabolism remains a major challenge (Wang Mittelheuser and van Steveninck (1969) in the United and Blatt, 2011; Horrer et al., 2016; Griffiths and Males, States discovered ABA to be highly effective in closing 2017; Santelia and Lunn, 2017). Considerable attention, stomata and in the subsequent resistance of the leaf to too, has been drawn in recent years to the unusual wilting. This same period, during the 1960s and 1970s, pattern of stomatal development within the epidermis marked a recognition of ion transport, especially of K+ and to its evolution (McElwain et al., 2005; Bergmann salts, and of solute content contributing to the cell tur- and Sack, 2007; Chen et al., 2017). Thus, present interest gor as a driver behind stomatal movements (Fischer in stomata extends well beyond ion transport and gas and Hsiao, 1968; Humble and Hsiao, 1969). Ironically, exchange. Some of these topics are explored in depth in research on stomatal movements at the time was mo- this Focus Issue, and we direct the reader to the several tivated by interest in the mechanism of opening and by Updates accompanying this article (Brodribb and the new concepts of chemiosmosis (Mitchell, 1969). McAdam, 2017; Chater et al., 2017; Vialet-Chabrand Opening, but not closing, was thought to be active, re- et al., 2017). Nonetheless, in many respects, the focus quiring coordination and energy for transport. Re- has come full circle, returning to issues of membrane search came to focus on stomatal closure only following transport and its control. If we are to use our knowledge MacRobbie’s pioneering radiotracer flux analysis in the of stomata to improve crop resilience and agricultural 1980s. Her studies showed that ion efflux during clo- capacity in marginal areas, then stomatal gas exchange sure is a highly coordinated process (MacRobbie, 1981, (Buckley, 2017; Franks et al., 2017) must be linked to an 1983a). understanding of the mechanics of stomatal ion trans- The last three decades have seen an explosion in re- port and its regulation as a priority for the future. Here, search directed to the mechanics of solute transport and we review the current knowledge of ion transport in its regulation. The majority of this new knowledge stomatal guard cells. We emphasize its dynamics and comes from electrophysiological studies, both voltage coordination, the origins of which often defy intuitive clamp on intact stomatal guard cells and patch clamp understanding yet are critical to any rational efforts on guard cell protoplasts, that allow separate transport toward stomatal engineering, and we stress the im- activities to be identified and characterized. These ef- portance of quantitative functional data that are es- forts have provided an unprecedented depth of quan- sential to realize such efforts. titative information about the kinetics of individual ion transporters, including those of the H+-ATPases, K+, 2 Cl , and Ca2+ channels at the plasma membrane and STOMATAL OPENING several cation- and anion-selective channels at the tonoplast, and about the dynamics of their regulation Stomatal pores form between specialized pairs of (Pandey et al., 2007; Sokolovski and Blatt, 2007; Kim epidermal cells, the guard cells (Fig. 1). Guard cells of et al., 2010; Roelfsema and Hedrich, 2010; Lawson dicotyledonous plants bow apart as they expand, and Blatt, 2014). With the cloning of many of these thereby opening the stomatal pore (Table I). The 488 Plant Physiol. Vol. 174, 2017 Downloaded from on July 10, 2017 - Published by www.plantphysiol.org Copyright © 2017 American Society of Plant Biologists. All rights reserved. Guard Cell Ion Transport malate (Mal), as well as sugars. In closing the pore, guard cells reverse this process by metabolizing these solutes or releasing them to the apoplast. The changes in solute content between the open and closed states 2 are substantial, often exceeding 300 to 400 mosmol L 1 (= 300–400 mosM) on a cell volume basis (Table II), and draw corresponding water fluxes, thereby driving the volume and turgor changes that open and close the pore.